Method for fabricating a semiconductor component based on GaN

ABSTRACT

A semiconductor component has a plurality of GaN-based layers, which are preferably used to generate radiation, produced in a fabrication process. In the process, the plurality of GaN-based layers are applied to a composite substrate that includes a substrate body and an interlayer. A coefficient of thermal expansion of the substrate body is similar to or preferably greater than the coefficient of thermal expansion of the GaN-based layers, and the GaN-based layers are deposited on the interlayer. The interlayer and the substrate body are preferably joined by a wafer bonding process.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation of copending InternationalApplication No. PCT/DE01/03851, filed Oct. 8, 2001, which designated theUnited States and was not published in English.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The invention relates to a method for fabricating a semiconductorcomponent having a plurality of GaN-based layers.

[0004] Semiconductor components based on GaN are used predominantly togenerate radiation in the blue-green spectral region and have aplurality of layers that are formed of a GaN-based material. In additionto GaN itself, materials of this type also include materials derivedfrom GaN or related to GaN and also ternary or quaternary mixed crystalsbuilt up on the basis of this structure. These materials include, inparticular, AlN, InN, AlGaN (Al_(1−x)Ga_(x)N, 0≦x≦1), InGaN(In_(1−x)Ga_(x)N, 0≦x≦1), InAlN (In_(1−x)Al_(x)N, 0≦x≦1) and AlInGaN(Al_(1−x−y)In_(x)Ga_(y)N, 0≦x≦1, 0≦y≦1). In the text that follows, theterm “GaN-based” relates to these materials systems as well as GaNitself.

[0005] Epitaxy processes are usually used to fabricate GaN-basedsemiconductor components. The choice of an epitaxy substrate is ofcrucial importance both to the fabrication process and to the functionof the component.

[0006] Sapphire or SiC substrates are often used for this purpose, butboth entail certain drawbacks. For example, the lattice mismatch ofsapphire with respect to GaN-based layers is relatively high.

[0007] In this respect, SiC substrates have better lattice matching toGaN-based materials. However, the fabrication of SiC substrates withsufficient crystal quality is associated with very high costs. Moreover,the yield of GaN-based semiconductor components is relatively low, sincethe size of SiC wafers is limited to diameters of typically well below150 mm.

[0008] U.S. Pat. No. 5,786,606 discloses a method for fabricatingradiation-emitting semiconductor components based on GaN in which an SiClayer is grown epitaxially on a separation by implantation of oxygensubstrate (SIMOX substrate) or an silicon on insulator (SOI) substrate.Then, a plurality of GaN-based layers are deposited on the SiC layer.

[0009] However, the SiC layer reduces the radiation output of thecomponent, since some of the radiation generated is absorbed in the SiClayer. Furthermore, the epitaxial formation of a SiC layer withsufficient crystal quality also requires a high level of fabricationoutlay.

SUMMARY OF THE INVENTION

[0010] It is accordingly an object of the invention to provide a methodfor fabricating a semiconductor component based on GaN that overcomesthe above-mentioned disadvantages of the prior art methods of thisgeneral type, which is technically simple and inexpensive. The inventionfurther relates to fabricating semiconductor components having anincreased radiation output.

[0011] With the foregoing and other objects in view there is provided,in accordance with the invention, a method for an epitaxial fabricationof a semiconductor component. The method includes providing a compositesubstrate having a substrate body with a given coefficient of thermalexpansion, and an interlayer, and applying GaN-based layers to theinterlayer of the composite substrate. The given coefficient of thermalexpansion of the substrate body being equal to or greater than acoefficient of thermal expansion of the GaN-based layers.

[0012] In the fabrication method according to the invention, a pluralityof GaN-based layers are applied epitaxially to a composite substratewhich includes a substrate body and an interlayer, the coefficient ofthermal expansion of the substrate body being similar to or greater thanthe coefficient of thermal expansion of the GaN-based layers.

[0013] In a plurality of GaN-based layers of different composition, thecoefficient of thermal expansion of the layers also differs. However,these deviations are generally minor and are negligible compared to thedifference from the coefficient of thermal expansion of the substratebody. The crucial coefficient of thermal expansion of the GaN-basedlayers is primarily the coefficient of thermal expansion of the layerthat adjoins the composite substrate. Furthermore, depending on thestructure of the layer sequence, the coefficient of thermal expansion ofthe GaN-based layer which has the greatest thickness or the mean of thecoefficients of thermal expansion, if appropriate weighted according tothe respective layer thicknesses, can also be used for this purpose.

[0014] In the invention, the coefficient of thermal expansion of thesubstrate body is greater than or similar to the coefficient of thermalexpansion of the GaN-based layers. In the latter case, the coefficientof thermal expansion of the substrate body preferably deviates by nomore than 50%, particularly preferably by no more than 30%, from theexpansion coefficient of the GaN-based layers.

[0015] A composite substrate is to be understood as meaning a substratethat includes at least two regions, the substrate body and theinterlayer, and as such forms the starting substrate for the epitaxyprocess. In particular, the interlayer is applied to the substrate bodynot by epitaxy but rather preferably by a bonding process.

[0016] A suitable bonding process is preferably an oxidic bondingprocess or a wafer bonding process. In the case of oxidic bonding,substrate body and interlayer are joined to one another by the formationof an oxide layer, for example a silicon oxide layer, as a bondinglayer, while in the case of wafer bonding the substrate body and theinterlayer are joined to one another directly. Furthermore, it is alsopossible to use other bonding processes, for example eutectic bondingprocesses or bonding processes in which a nonoxidic bonding layer isformed.

[0017] With a composite substrate of the type described, the thermalproperties are determined primarily by the substrate body, while,substantially independently of this, the epitaxy surface and inparticular its lattice constant are defined by the interlayer. As aresult, the interlayer can advantageously be optimally matched to thelattice constant of the layers that are to be applied. At the same time,the use of a substrate body with a sufficiently high coefficient ofthermal expansion prevents tensile distortion to the GaN-based layersduring the cooling phase after they have been applied, which wouldresult in the formation of cracks in the layers. Therefore, theinterlayer is preferably configured to be so thin that the coefficientof thermal expansion of the composite substrate as a whole substantiallycorresponds to the expansion coefficient of the substrate body. Thesubstrate body is typically at least twenty times thicker than theinterlayer.

[0018] In an advantageous configuration of the invention, the substratebody contains SiC, Si or GaN, preferably polycrystalline (poly-SiC,poly-Si or poly-GaN), sapphire or AlN. The coefficient of thermalexpansion of SiC is similar to the expansion coefficient of GaN-basedmaterials, while the other materials mentioned have a higher coefficientof thermal expansion than GaN-based materials. This advantageouslyavoids the formation of cracks during cooling of the epitaxially appliedlayers.

[0019] In a preferred configuration of the invention, the interlayercontains SiC, silicon, sapphire, MgO, GaN or AlGaN. These materials areparticularly suitable for forming a substantially monocrystallinesurface with a lattice constant which is matched to GaN. The epitaxysurface used is preferably a Si(111) surface or a monocrystalline SiCsurface on which the GaN-based layers are grown.

[0020] In an advantageous refinement of the invention, the GaN-basedlayers are deposited on a composite substrate in which the interlayerhas been applied to the substrate body by a bonding process, for examplea wafer bonding process or an oxidic bonding process. It is preferablefor a bonding layer, for example of silicon oxide, to be formed betweensubstrate body and interlayer.

[0021] A bonding process advantageously allows a wide range of materialssystems to be combined without having to be constrained byincompatibility between the materials, as occurs, for example, when aninterlayer is applied epitaxially to a substrate body.

[0022] To obtain a sufficiently thin interlayer, it is also possible fora thicker interlayer to be bonded to the substrate body and then to bethinned to the thickness required, for example by grinding or splitting.

[0023] In an advantageous refinement of the invention, before theGaN-based layers are deposited on the composite substrate, a mask layeris formed, with the result that the GaN-based layers only grow on theregions of the epitaxy surface that are not covered by the mask. As aresult, the GaN-based layers are advantageously interrupted in the layerplane, resulting in additional protection against tensile distortion andthe associated formation of cracks.

[0024] A further preferred configuration of the invention consists inpatterning the GaN-based layers into individual semiconductor layerstacks after they have been deposited on the composite substrate. Then,a carrier is applied to the GaN-based semiconductor layer stacks, andthe composite substrate is removed. The composite substrate cantherefore be reused at least in part. This constitutes a particularadvantage in the case of SiC substrate bodies, the fabrication of whichentails very high costs. Furthermore, in this way a thin-film componentis fabricated. In this context, a thin-film component is to beunderstood as meaning a component which does not include an epitaxysubstrate.

[0025] In this way, in the case of radiation-emitting semiconductorcomponents, the radiation output is increased, since absorption of thegenerated radiation in the epitaxy substrate, as occurs in particularwith SiC substrates, is avoided.

[0026] Examples of suitable materials for the carrier include GaAs,germanium, silicon, zinc oxide or metals, in particular molybdenum,aluminum, copper, tungsten, iron, nickel, cobalt or alloys thereof.

[0027] The carrier material is preferably selected in such a way thatits coefficient of thermal expansion is matched to the coefficient ofthermal expansion of the GaN-based layers and if appropriate to thecoefficient of thermal expansion of the substrate body. It is expedientfor the coefficient of thermal expansion of the carrier material to bematched to the coefficient of thermal expansion of the substrate body inparticular if the temperature is changed between the application of thecarrier and the removal of the GaN-based layers from the compositesubstrate. Very divergent coefficients of thermal expansion would leadto considerable expansion of carrier and composite substrate and therebyincrease the risk of damage to the GaN-based layers between them as aresult of excessive mechanical stresses.

[0028] It is advantageous to match the coefficients of thermal expansionof carrier and GaN-based layers in order to keep mechanical stresses,which may occur on the one hand after fabrication of the semiconductorbodies during a cooling phase and on the other hand in operation, forexample as a result of heating through power losses, at a low level.

[0029] Matched coefficients of thermal expansion are in particularcharacterized by the difference between them being so low that thetemperature changes which occur cause substantially no damage to theGaN-based layers as a result of thermally induced mechanical stresses.The relative deviation of the coefficient of thermal expansion of thecarrier from the coefficient of thermal expansion of the compositesubstrate should preferably be less than 50%, particularly preferablyless than 30%.

[0030] The temperature changes which occur are caused, for example, bythe process used to separate the GaN-based layers from the compositesubstrate, the temperature which prevails during fabrication, inparticular during application of the carrier, compared to the intendedoperating temperature, and/or the power loss which is to be expected onthe basis of the operating specifications.

[0031] The carrier material is preferably selected in such a way thatthe coefficient of thermal expansion of the carrier is between thecoefficient of thermal expansion of the substrate body and thecoefficient of thermal expansion of the GaN-based layers.

[0032] The coefficient of thermal expansion of the carrier isparticularly preferably greater than the arithmetic mean of thecoefficients of thermal expansion of composite substrate and GaN-basedlayers.

[0033] The so-called transfer bonding of the semiconductor layer stacksfrom the composite substrate to a carrier which is described may also,according to the invention, take place in two steps, in which case theGaN-based semiconductor layer stacks are bonded to a temporary carrierand are then bonded to the actual carrier, so that ultimately the actualcarrier replaces the composite substrate. Semiconductor layer stacksfabricated in this way advantageously have a corresponding layersequence to GaN-based semiconductor bodies with epitaxy substrate inaccordance with the prior art, so that the same subsequent processsteps, such as for example separation, contact-making and installationin a housing, can be used for both layer stacks.

[0034] In a particularly preferred refinement of the method forfabricating radiation-emitting semiconductor bodies based on GaN, areflector layer is formed on the semiconductor layer stack in order toincrease the radiation output. The radiation output of GaN-basedsemiconductor components is largely limited by reflection at thesemiconductor body interfaces, on account of the high refractive indexof GaN-based materials. In the case of radiation-emitting semiconductorbodies without an absorbent substrate, it is advantageously possible forthe radiation components that are reflected at the output surface to bereturned to the output surface by a reflector layer. This furtherincreases the radiation output.

[0035] The reflector layer is preferably formed as a metal layer whichcontains, for example, aluminum, silver or a corresponding aluminum orsilver alloy.

[0036] A metal layer of this type can advantageously also be used as acontact surface. Alternatively, the reflector layer may also be formedby a dielectric mirror coating in the form of a plurality of dielectriclayers.

[0037] In an advantageous refinement of the invention, at least part ofthe surface of the semiconductor layer stack is roughened. Thisinterferes with total reflection at the surface and thereby increasesthe radiation output. The roughening is preferably effected by etchingor a sand-blasting process.

[0038] Other features which are considered as characteristic for theinvention are set forth in the appended claims.

[0039] Although the invention is illustrated and described herein asembodied in a method for fabricating a semiconductor component based onGaN, it is nevertheless not intended to be limited to the details shown,since various modifications and structural changes may be made thereinwithout departing from the spirit of the invention and within the scopeand range of equivalents of the claims.

[0040] The construction and method of operation of the invention,however, together with additional objects and advantages thereof will bebest understood from the following description of specific embodimentswhen read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0041] FIGS. 1A-1G are diagrammatic, sectional views through a firstexemplary embodiment of a fabrication method according to the invention;

[0042] FIGS. 2A-2 i are diagrammatic, sectional views through a secondexemplary embodiment of a fabrication method according to the invention;and

[0043] FIGS. 3A-3C are diagrammatic, sectional views through a thirdexemplary embodiment of a fabrication method according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0044] In all the figures of the drawing, sub-features and integralparts that correspond to one another bear the same reference symbol ineach case. Referring now to the figures of the drawing in detail andfirst, particularly, to FIGS. 1A-1G thereof, there is shown afabrication method. A composite substrate has a substrate body 1 madefrom poly-SiC to which a monocrystalline SiC interlayer 2 has beenbonded in a known way is used. For this bonding, a bonding layer 3, forexample of silicon oxide, has been formed between the substrate body 1and the interlayer 2, see FIG. 1A.

[0045] A plurality of GaN-based layers 4 are grown epitaxially onto thecomposite substrate, FIG. 1B. The structure of the layer sequence is notin principle subject to any particular restrictions.

[0046] In this case, it is preferable to form an active layer that isused to generate radiation and is surrounded by one or more sheathinglayers and/or waveguide layers. The active layer may in this case beformed by a plurality of thin individual layers in the form of a singleor multiple quantum well structure.

[0047] Furthermore, it is advantageous to form a buffer layer, forexample based on AlGaN, on the interlayer 2, making it possible toimprove the lattice matching and wettability with respect to thesubsequent layers. To increase the electrical conductivity of a bufferlayer of this type, electrically conductive passages, for example basedon InGaN, may be included in the buffer layer.

[0048] Then, the GaN-based layers 4 are divided into individualsemiconductor layer stacks 5 by lateral patterning, preferably by mesaetching, FIG. 1C.

[0049] In the next step, FIG. 1D, a carrier 6, for example made fromGaAs or a material which transmits the radiation which is generated, isapplied to the semiconductor layer stacks 5.

[0050] Then, the composite substrate 1, 2, 3 including the interlayer 2is removed from the semiconductor layer stacks 5, FIG. 1E. This can beachieved, for example, by an etching process in which the interlayer 2or the bonding layer 3 is destroyed. Furthermore, the compositesubstrate can also be removed by a laser ablation process, in which casea substrate body that transmits the laser radiation used, for example asapphire substrate body, is expediently used. The laser radiation canthereby be radiated through the substrate body onto the interlayer orthe bonding layer. The substrate body 1 can advantageously be reused ina further fabrication cycle.

[0051] If the temperature is changed between the application of thecarrier and the removal of the composite substrate, it is particularlyexpedient to match the coefficients of thermal expansion of the carrierand the substrate body. By way of example, in combination with asapphire substrate body, a suitable carrier contains GaAs, molybdenum,tungsten or an Fe—Ni—Co alloy. By way of example, a eutectic bondingprocess can be used to apply a metallic carrier.

[0052] In combination with a SiC substrate body, a material thatcontains silicon or SiC, in each case in monocrystalline or preferablypolycrystalline form, is an advantageous carrier material. In thiscontext, by way of example, an oxidic bonding process is suitable forapplication of the carrier.

[0053] Then, contact surfaces 10 are applied to the thin-filmsemiconductor bodies 5 formed in this way, FIG. 1F. Finally, thesemiconductor layer stacks 5 are separated from one another, FIG. 1G,and processed further in the usual way.

[0054] In the fabrication method illustrated in FIGS. 2A-2 i, thestarting point is once again the composite substrate which issubstantially formed by the poly-SiC substrate body 1 and the Si(111)interlayer 2. The interlayer 2 has been applied to the substrate body 1with the aid of an oxidic bonding process to form the silicon oxidebonding layer 3, FIG. 2A. Alternatively, the substrate body 1 and theinterlayer 2 may also be joined by another bonding process, for examplewafer bonding.

[0055] Once again, a plurality of GaN-based layers 4 are grown onto thecomposite substrate, FIG. 2B, and finally these layers are provided witha contact layer 8, for example of platinum, FIG. 2C.

[0056] Then, the GaN-based layers 4 are divided into individualsemiconductor layer stacks 5 by etch patterning, FIG. 2D.

[0057] For protection purposes, a passivation layer 11, preferably basedon silicon nitride, is applied to the semiconductor layer stacks 5 whichhave been formed in this way, FIG. 2E.

[0058] Then, a bonding solder 12 followed by a reflector 9 containing asilver or aluminum alloy are deposited on the regions of the contactlayer 8 which are not covered by the passivation layer, FIG. 2F.

[0059] Then, the semiconductor layer stacks 5 with the reflector 9 aretransfer-bonded to the carrier 6 by use of a eutectic bonding process,FIG. 2G.

[0060] In the next step, FIG. 2H, the substrate body 1 is removed andcan thereby be reused.

[0061] Finally, the top side of the individual semiconductor layerstacks is provided with contact surfaces 10, FIG. 2I. The semiconductorlayer stacks can then be separated from one another and if appropriatefitted into non-illustrated housings.

[0062] The exemplary embodiment of a fabrication method according to theinvention that is illustrated in FIGS. 3A-3C represents a variant on theexemplary embodiments described above.

[0063] Once again, as described above, the epitaxy substrate used is acomposite substrate, FIG. 3A.

[0064] Prior to the deposition of the GaN-based layers 4, a mask layer 7is applied to the epitaxy surface of the interlayer 2, FIG. 3B.Consequently, the GaN-based layers 4 only grow on those regions of theepitaxy surface which are not covered by the mask layer 7 (epitaxywindows), FIG. 3C. As a result, the GaN-based layers 4 are interruptedin the direction of the layer plane. This additionally avoids tensilestresses in the epitaxially deposited layers during the cooling phase.

[0065] The fabrication method can then be continued as in the otherexemplary embodiments.

[0066] Of course, the explanation of the invention that has been givenon the basis of the exemplary embodiments described is not to beunderstood as constituting any limitation to the invention, but ratherthe invention encompasses all embodiments that make use of the inventiveidea.

We claim:
 1. A method for an epitaxial fabrication of a semiconductorcomponent, which comprises the steps of: providing a composite substratehaving a substrate body with a given coefficient of thermal expansion,and an interlayer; and applying GaN-based layers to the interlayer ofthe composite substrate, the given coefficient of thermal expansion ofthe substrate body being equal to or greater than a coefficient ofthermal expansion of the GaN-based layers.
 2. The method according toclaim 1, which further comprises setting a thickness of the interlayersuch that a coefficient of thermal expansion of the composite substrateis substantially determined by the substrate body.
 3. The methodaccording to claim 1, which further comprises forming the substrate bodyfrom a material selected from the group consisting of SiC, poly-SiC, Si,poly-Si, sapphire, GaN, poly-GaN and AlN.
 4. The method according toclaim 1, which further comprises forming the interlayer from a materialselected from the group consisting of SiC, Si, sapphire, MgO, GaN andAlGaN.
 5. The method according to claim 1, which further comprisesforming the interlayer with a monocrystalline surface at least inpartial regions.
 6. The method according to claim 1, which furthercomprises forming the substrate body from poly-SiC and the interlayerfrom monocrystalline SiC.
 7. The method according to claim 1, whichfurther comprises forming the substrate body from poly-Si and theinterlayer from monocrystalline Si.
 8. The method according to claim 1,which further comprises forming the substrate body from poly-GaN and theinterlayer from monocrystalline GaN.
 9. The method according to claim 1,which further comprises: forming the interlayer with one of an Si(111)surface and an SiC surface which is monocrystalline at least in partialregions; depositing the GaN-based layers on one of the Si(111) surfaceand the SiC surface.
 10. The method according to claim 1, which furthercomprises applying the interlayer to the substrate body using a bondingprocess.
 11. The method according to claim 1, which further comprisesforming a bonding layer between the substrate body and the interlayer.12. The method according to claim 11, which further comprises formingthe bonding layer from silicon oxide.
 13. The method according to claim1, which further comprises forming a mask layer with epitaxy windowsbefore the GaN-based layers are applied to the composite substrate, anepitaxy surface of the composite substrate within the epitaxy windowsremaining uncovered.
 14. The method according to claim 1, which furthercomprises patterning the GaN-based layers into individual semiconductorlayer stacks after the GAN-based layers have been applied to thecomposite substrate.
 15. The method according to claim 14, which furthercomprises: applying a carrier to the semiconductor layer stacks; andremoving the composite substrate.
 16. The method according to claim 14,which further comprises: applying a temporary carrier to thesemiconductor layer stacks; removing the composite substrate; applying acarrier to that side of the semiconductor layer stacks from which thecomposite substrate has been removed; and removing the temporarycarrier.
 17. The method according to claim 15, which further comprisesforming the carrier from a compound or element selected from the groupconsisting of GaAs, germanium, silicon, zinc oxide, molybdenum,aluminum, copper, iron, nickel, and cobalt.
 18. The method according toclaim 17, which further comprises forming the substrate body fromsapphire and the carrier from a material selected from the groupconsisting of GaAs, molybdenum, tungsten, and an Fe—Ni—Co alloy.
 19. Themethod according to claim 17, which further comprises forming thesubstrate body from SiC and the carrier from silicon or SiC.
 20. Themethod according to claim 15, which further comprises matching acoefficient of thermal expansion of the carrier to the coefficient ofthermal expansion of the GaN-based layers.
 21. The method according toclaim 15, which further comprises matching a coefficient of thermalexpansion of the carrier to the given coefficient of thermal expansionof the substrate body.
 22. The method according to claim 15, whichfurther comprises forming the carrier to have a coefficient of thermalexpansion to be between the given coefficient of thermal expansion ofthe substrate body and the coefficient of thermal expansion of theGaN-based layers.
 23. The method according to claim 14, which furthercomprises forming a reflector layer on one of the GaN-based layers andthe semiconductor layer stacks.
 24. The method according to claim 23,which further comprises forming the reflector layer by applying a metallayer.
 25. The method according to claim 24, which further comprisesforming the metal layer from a material selected from the groupconsisting of silver, aluminum, silver alloy, and aluminum alloy. 26.The method according to claim 23, which further comprises simultaneouslyusing the reflector layer as a contact surface.
 27. The method accordingto claim 23, which further comprises forming the reflector layer from adielectric mirror coating.
 28. The method according to claim 14, whichfurther comprises roughening a surface of the semiconductor layer stacksat least in regions.
 29. The method according to claim 28, which furthercomprises etching a surface of the semiconductor layer stacks forroughening the semiconductor layer stacks.
 30. The method according toclaim 28, which further comprises roughening a surface of thesemiconductor layer stacks by performing a sand-blasting process. 31.The method according to claim 1, which further comprises applying theinterlayer to the substrate body using a bonding process selected fromthe group consisting of an oxidic bonding process and a wafer bondingprocess.
 32. A thin-film semiconductor component selected from the groupconsisting of radiation-emitting components, diodes, transistors,radiation-emitting diodes, LEDs, semiconductor lasers andradiation-detecting components produced according to the method of claim15.
 33. A thin-film semiconductor component selected from the groupconsisting of radiation-emitting components, diodes, transistors,radiation-emitting diodes, LEDs, semiconductor lasers andradiation-detecting components produced according to the method of claim16.
 34. A method of using a composite substrate having a substrate bodyand an interlayer for an epitaxial fabrication of a semiconductorcomponent having a plurality of GaN-based layers, which comprises thestep of: joining the substrate body to the interlayer using a bondingprocess.
 35. The method according to claim 34, which further comprisesjoining the substrate body to the interlayer using one of an oxidicbonding process and a wafer bonding process.